Group III nitride semiconductors have a direct band gap of energy corresponding from the visible light range to the ultraviolet range and are capable of high efficiency light emission, and they are therefore used in products such as light emitting diodes (LED) and laser diodes (LD). In particular, the realization of white light emitting diodes by combination with fluorescent materials is anticipated as a new field of application for light emitting diodes.
The output of a light emitting diode is determined by the product of the internal quantum efficiency dependent on the epitaxial layer structure and crystallinity, and the light extraction efficiency dependent on reabsorption within the device and device shape. Several methods of working device shapes are known for increasing light extraction efficiency (Japanese Utility Model Application Publication No. 51-142870, and Japanese Unexamined Patent Publication No. 56-50586).
Output can be improved by the same principle in nitride semiconductors as well, and increased light emission output of devices can be achieved by shape working of devices in the same manner (Japanese Unexamined Patent Publication Nos. 2004-6662 and 2004-87930).
High-quality nitride semiconductors are usually grown on sapphire (Al2O3) or silicon carbide (SiC) as the substrate, using MOCVD as the growth process. Sapphire and silicon carbide are selected because they are stable substances at high temperature, and are even stable at temperatures of 1000° C.-1200° C. employed for growth of nitride semiconductors by MOCVD.
However, nitride semiconductors in devices and sapphire or SiC used as substrates are also known to be hard substances that are difficult to work, and working of devices is accomplished by laser working, dry etching with plasma or high-temperature wet etching.
Laser working involves locally heating the working site to ultrahigh temperature and accomplishing working by ablasion and vaporization. This manner of working is advantageous because of the high working speed and high throughput. A disadvantage to be considered, however, is that the sample is subjected to high temperature in proximity to the working site.
Also, the working material that has scattered by the abrasion and vaporization tends to adhere back onto the wafer, thus often requiring some sort of etching treatment after laser working.
Other methods used for etching of nitride semiconductors employ harsh working conditions, and for example, particles with energy of a few tens of eV participate in the reaction in dry etching, but in terms of temperature this corresponds to a heat energy of several hundred thousand degrees, and therefore depending on the conditions the working sections may be exposed to several 100° C. Also, since in dry etching the working is carried out in an atmosphere with a halogen such as chlorine in an excited state, other sections are also affected during working of the element at the desired sections.
Laser working and etching treatment are particularly problematic when electrodes are formed on the element. The heat generated at the worked sections causes deterioration of the electrode surfaces when they are in proximity, thereby impairing the device characteristics. In addition, the halogen used for etching causes extensive corrosion of the electrodes which are composed mainly of metal, and if the electrode is not adequately protected with a mask it can itself become etched. In device working processes after electrode formation, it is necessary to examine the intended working conditions and select the conditions in consideration of their effect, creating the problem of a narrow process window.